Nature and Energy Redistribution of Highly Vibrationally Excited Polyatomic Molecules

• Main Author: Perry, Joseph Walter
• Format: Others
• Language: en
• Published: 1984
• Online Access: https://thesis.library.caltech.edu/1259/1/Perry_jw_1984.pdf; Perry, Joseph Walter (1984) Nature and Energy Redistribution of Highly Vibrationally Excited Polyatomic Molecules. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/YDWZ-Z639. https://resolver.caltech.edu/CaltechETD:etd-04032008-111559 <https://resolver.caltech.edu/CaltechETD:etd-04032008-111559>

<p>This thesis is a primarily experimental study of the spectroscopy and dynamics of highly vibrationally excited molecules in the ground and excited electronic states. Linear spectroscopic studies have been performed on CH-stretching overtones of medium and large sized molecules in the gas and solid phases. Also new time-resolved methods based on nonlinear uultiphoton ionization spectroscopy have been applied on the picosecond time-scale to the study of ultrafast intramolecular vibrational-energy redistribution (IVR) in isolated molecules.</p> <p>The first observation of high-energy CH stretching overtones of molecules in low-temperature solids is reported. Spectra of the ^νCH = 5 transitions of many aromatic molecules at ~2K reveal homogeneously broadened bands which are assigned to inequivalent local modes by studies of same isotopically substituted molecules. Typical linewidths give population lifetimes of ~50 fs for aromatic compared to 100-200 fs for methyl CHs in these large molecules. The polarization ratios of durene CH overtones are in good agreement with those predicted with the localized bond-mode transition moments.</p> <p>Overtone spectra of some gas-phase deuterated methanes and tetramethyl compounds have been studied. Spectra of CHD₃ have shown severe vibrational mixing at ^νCH = 6 and 7 with an apparent tuning of the Fermi-resonance between |^νCH&#62; and discrete stretch-bend combination states, e.g., |^(ν-1)CH, ²bend&#62;. Nonetheless, the lifetime of the CHD₃ overtones must be long (&#8805;5 ps) as evidenced by the narrow resolution limited linewidths. The splitting of the |6,0&#62;&#177; states of CH₂D₂ is inferred to be &#60;10 cm^-1 and shows that the coupling of the CH oscillators is relatively weak in agreement with theoretical prediction. In general, the results on the CH overtones indicate a significant role of CH stretch-normal mode coupling in the spectra and dynamics.</p> <p>Picosecond pump and probe multiphoton ionization is developed for the study of IVR in isolated molecules. Experimental evidence for IVR is presented. Nonexponential decay of vibronic states of S₁ trans-stilbene is analyzed in terms of a population rate equation and time-dependent Franck-Condon factor description of IVR. The thermally-averaged IVR time constant is about 2 ps. Also, the excess vibrational energy dependence of the S₁ trans-stilbene isomerization rate is determined for total vibrational energy of 2000 to 4500 cm^-1 and good agreement is found with calculated thermally averaged rates based on independent results on jet-cooled molecules.</p>